Wave filter of extended area



RELATIVE TRANSMISSION db.

F eb. 8, 1955 W. E. KOCK WAVE FILTER OF EXTENDED AREA Filed June 25, 1950 FIG. l

FREOUENCY- KC.

A TOR/ver United States Patent O WAVE FILTER E EXTENDED AREA Winston E. Kock, Basking Ridge, N. J., assignor to Bell Telephone Laboratories, Incorporated, New York, N. Y., a corporation of New York Application June 23, 1950, Serial No. 169,967

3 Claims. (Cl. 181-33) This invention relates to filters for waves of various kinds and more particularly to those filter structures which are suitable for acting upon waves passing through relatively large areas in space as distinguished from structures of constricted cross-sectional area. Electromagnetic waves as well as acoustic or other compressional and elastic waves are contemplated as being adapted to be filtered by structures embodying the invention.

In many applications particularly of compressional waves of audible frequency or of frequencies either above or below audibility it is desirable to provide a filtering of certain unwanted frequencies so as to eliminate them from an admixture with the desired signal or other desired transmission. Filtering devices adapted to perform this function have usually been placed in the electrical circuit portion of a system using electrical transmission in conjunction with acoustic transmission. It is also known, however, to perform mechanical filtering of the compressional wave itself. Devices in this latter category have generally been of the nature of resonant cavities embodied in generally tubular structures of relatively restricted cross-section so that the compressional wave to be filtered must be passed through a single tube of constricted cross-section.

In accordance with the present invention a type of filtering structure is employed which can be made relatively very large in extent and may thus act upon waves passing through a space of relatively large cross-sectional area as compared with the constricted cross-section of a tubular structure.

One application of the invention is in conjunction with high fidelity radio receivers where there is a beat note, e. g. of -kilocycle frequency, between radio stations, which beat note appears as a whistling sound in the reproduced program and is objectionable.

While the structures disclosed herein, which are illustrative of the invention, are primarily designed for use with compressional or elastic waves, the identical or similar structures may in many cases be used with electromagnetic waves of equal or nearly equal wavelength. As the operation of the structures is believed to be dependent upon the efficient reflection of waves from the surfaces of the structures serving as elements of the filters, it will be evident that for use with compressional or elastic waves, a structure requires the necessary mechanical rigidity to reflect compressional Waves from its surfaces and has no need of electrical conductivity, whereas the same or any other structure for use with electromagnetic waves requires the necessary surface conductivity to reflect electromagnetic waves effectively and requires only enough mechanical rigidity to maintain its parts in the desired positions relatively to one another.

I wish it to be understood that the embodiments of the invention shown herein are illustrative only and do not limit the kinds of structure which may be employed within the spirit and scope of the invention.

In the drawing:

Fig. l is a perspective view of an embodiment of the invention in conjunction with a loudspeaking telephone receiver;

Fig. 2 is a detailed dimensional drawing of a crosssection of the structure of Fig. l along the line 2 2;

Fig. 3 is an elevational view of a radio receiver embodying the invention;

Fig. 4 is a schematic drawing of a transmission measuring system for determining the transmission vs. frequency characteristic of a filter structure as shown in Fig. 1

2,701,617 Patented Feb. 8, 1955 Fig. 5 is a representative transmission vs. frequency characteristic of a filter structure as determined in the measuring system of Fig. 4;

Figs. 6 and 7 are fragmentary perspective views of alternative structures embodying the invention; and

Fig. 8 shows the structure of Fig. 7 with certain elements rotated with respect to their positions as shown in Fig. 7.

Fig. 1 shows a filter l0, in accordance with the invention, placed in proximity to a cone-type dynamic loudspeaking telephone 11, the latter having electrical input lead wires 12. The filter elements proper are in the form of rigid channel bars such as 13 and 14 rigidly mounted in a frame 15 which may be of wood or other suitable supporting material.

A filter which was built and successfully operated in suppressing a 10,000-cycle whistle caused by interference between carrier frequencies of adjacent broadcasting channels was built like the device 10 of Fig. l, of metal channel bars of cross-sectional dimensions as shown in Fig. 2 and spaced as shown therein.

The channel bars were of metal of 1g2-inch thickness with the inner dimensions of the resonant cavity comprising L9Ati-inch opening and 1t-inch depth. The bars were spaced with 1A inch space between, that is, they were spaced 1/2 inch apart on centers. The number and length of the bars were suiiicient to occupy an area at least as great as the area of the aperture of the loudspeaking receiver. The length and number of channels can be made as large as desired as, for example, if it is required that the structure be placed in a very large opening such as a door. The channels become resonant for waves having a wavelength equal to approximately four times the depth of the channel. When they become resonant they so react as to cause the air in their immediate vicinity to partake in the motion of air within the channel. Under these conditions, even though there might be a relatively large space between channels, the array acts like a solid sheet in so far as the reflection of waves at the critical frequency is concerned and provided that the spacing be not more than about one-half wavelength at the resonant frequency. The result is that standing Waves of the resonant frequency are set up in the space between the receiver and the filter and a high impedance is imparted to the electrical circuit of the receiver due to the receiver being driven by the standing waves in phase opposition to electrical waves incoming to the receiver from the electrical circuit. On the side of the filter 10 opposite from the receiver 11 waves of the resonant frequency are not built up to any appreciable extent.

The term rigid as applied to the material of which the channel bar is made and as applied to the mounting of the bars means rigid in the sense that the bar when so supported in substantially invariable in shape, size and position under the application of the forces exerted by the waves to be filtered. In other words the material does not vibrate in response to the forces impressed by the wave.

The cross-sectional shape of the bar need not be rectangular but may be any shape capable of resonance at the desired frequency.

Fig. 3 shows the structure of the device 10 embodied as a grille in front of the dynamic loudspeaker in a conventional radio receiver. The back surfaces 16 of the channels 13, 14 are visible from the front of the receiver. Alternatively the channels may be covered by means of a cloth or an ornamental wooden grille or like known means. A conventional tuning scale is shown at 17 and control knobs at 18.

Fig. 4 shows in schematic form a system useful in determining the transmission vs. frequency characteristic of a filtering device such as 10. A sound source 19,*such as a loudspeaker is mounted facing the device 10 at a suitable distance, such as l() feet. A feed horn 20 is mounted a short distance back of the device 10; in the case illustrated 35/8 inches. A microphone 21 is provided inside the horn 20, the microphone having electrical output leads 22.

In the operation of the system of Fig. 4, the loudspeaker is operated to emit substantially pure waves of "adjustable frequency. Inthe case of audible frequencies the Wavesemitted are` substantial-ly` pure` acousticn tones.

The leads 22 are connected to a suitable detector.'

Using a succession of input frequencies, the electrical outputof the microphone 21 -is measured at each fre- -quency,.ftrher hornV 20 and` microphone 21- together `cornprising a form of acoustic pick-up.

`Fig. `5.shows the result of a.v series of transmission vs. frequencynmeasurements made uponwthe-device 10 as dimensioned in Fig. 2, using the system of Fig. 4 for the measurements. The observed transmission l frequency characteristic is shown at 23. A comparison characteristic Withvthe filter removed from `the system is shown at 24. .The curve-23shows a pronounced minimum transmission` at av'wavelength of 3.24 centimeters, corresponding to a frequencyof about 10,400 cycles.

ffFrom Fig. `it is.seenthat vthe array of channels-is relatively transparent to waves of frequencies up to kilocycles, relatively opaque atfrequencies between 10 and 11 *kilocycles, `and` againtransparent -at frequencies .from ll` kilocycles up to 16 kilocycles. This property of opacitylin` a narrow` band offrequencies is, as has been stated `hereinbefore, due to. the resonancel of the channels .inthe vicinityof :this frequency band. The channels need not be oriented in any particular way. They may,-for example, have their channel opening faced in the direction of oncoming Waves, in a direction perpendicular thereto, in a direction away from the sources of the waves, or `in any otherv direction. Maximum reflection will occur :for Waves arriving in a direction perpendicular to the plane of the array. For waves arriving `at an angle of incidence oblique to the plane of the array, the effective depth of the resonator will not be a quarter wavelength at the same frequency as for perpendicular incidence. However, if the channels are oriented with their openings facing p'erpendicularly to the general vdirection of arrival, for example, pointing up or down in Fig. llinstead of toward 'the receiver as shown, then their transmission vs. frequency vcharacteristic is independent ofthe angle ofincidence of the Waves. y

It should be noted that an end effectmay cause the resonance of the channel bars to depart from the frequency at' which the'channel is one-quarter wavelength deep. A correction 'may be made for this end effect-in computing the resonant frequency. Such correction is appreciable in case the channel resonators have fairly wide openings. Reference may be vmade to Rayleighs .Theory of Sound, volume-2, page 487 (Appendix A) for a 'discussionof the end effect. The end correction is in the` nature'of' an ncrernent'added to the actual depth of the channel, that is, resonance is usually observed at afrequencyfor which the wavelength is shorter than four times thechannel depth.

Figf shows an alternative filtering structure, using a plurality of hollow spherical resonators spaced apart by rigid rods 26. For use with compressional waves, each resonator should have la rigid wall and an aperture 27' for thepurpose `oflproviding access of wavesto the interior of theresonator. The length of the rodsshould besuch'as to keep thespacing between resonators 'small as `compared to the resonant wavelength. In general, Vall thelresonators should'be tuned to substantially the same resonant frequency. The resonators`25 'may be arranged in a two-dimensional array or in a three-dimensional array, to'fit various requirements `ofspace and degree of filtering.

Fig. 7 shows another alternative filtering structure, using "a plurality rof half-wavelength cylindrical cavity resonators 28, open at both ends, spaced apart a quarter Wavelength on centers and arranged in staggered rows mounted upon rods v29, 30. Any suitable means may be provided for rotatingthe rods 29, 30, as for example by disks 31, 32 and a push-pull rod 33. By means of the rod 33 the amount of filtering exerted'by the array of cylinders 28 may be adjusted. The cylinders 28 exert nofilterin'g actionupon Waves progressing in the direction 0f thecentral longitudinal axis `of the cylinders and are most `effective upon Waves progressing perpendicularly to saidiaxis. lEach cylinder when its axis is perpendicular to the directionof `wave propagation reflects waves over aneffcctlve area approximately `one-half wavelength long by one-quarter wavelength wide, this area lying also per- .fpendiculanto-thef-direetionfof waveHpropagat-ion. 4As

shown in Fig. 7 the array of cylinders presents effectively a solid reflecting Wall to Waves of the resonant frequency while allowing waves of other frequencies to pass through.

Fig. 8 shows the structure of Fig. 7 with the cylindrical elements rotated with respectfto'their positions shown in Fig. 7. To a wave progressing in the direction perpendicular tol the paper the array Vasshownin`1?ig."8 is less Leffective as an absorber or reflector than lthe array as sh'own n`Fig.f7. -Ro`tatin of the cylinders1 28 intopan allelismL `with the direction of "wave propagation will" result in substantial annulment of the'filtering` effect.

It may be notedthatvthe directionalv polarization of the arrangement of"`Fi`gs. 7 and'8 is dependent upon the fact that both ends of the'cylinders `are open and there is unobstructed passageway for waves from one end to the other inside the cylinder. The polarization results from 4the wave strikingy the two `ends-vof the, cylinder` in the same or different phasesasthegcase may be. Ifthe cylinder were -divided by anl internalpartition the directional effectY would befabsentland the resonator Vwouldbe kunpolar-ized as inthe-case ofthefilter l10y of Fig.- 1.

Thcfilter/10 of Fig-1 is found to function without modification for electromagnetic -waves of; substantially the same wavelength, the frequency-l involved beingmhowever of an entirely different order 'of magnitud@ Similarly, theA structures of Figs. 6, 7` and y8 may be' usedwithout modification in 'the electromagnetic domain. "Modified'forms ofI the devices o f Figsfl, '6, 7 and 8 vwill be readily `devised bythosefskilled-in thevart, as Hwell as other and different forms within the spirit and scope of the invention.

What is claimed' is: l

1. A combinationgrillend'filter for the audio output orificeof `a radio'rece'iver to suppressan audio Vtonezgenerated by"interference betweenradio'waves received in said receiver fr'omtwo transmitting stations," comprising a' plural-ity? of charinel bars f sound reflective' material, the depthbf 'the channel being substantially "equal fto a quarter' wavelength4 of the audio` tone' 'to be" suppressed, said channel bars' beirgmount'ed parallel Hto one 'another in ak plane parallel to theplane of the orificeofftle radio receiver, the channelbars being jspa'eedapart "by open spaces 'therebetween which' are Iof 'material "widthnt exceeding'a halfw'avelength of-'the tone tobe' suppressed.

2. The combination 'according to claim l in j which the channel bars are'o'riented with'tlieir' open sides" facing the interior of the radio receiver.

3. A narrow-bandsupprcssionffilter comprising `lanfar ray of linearly extended channel-shaped cavity resonators, the depth .of the channelbeing substantially a q'uarter wavelength ofla wave'of the'principal frequency which the filter is designed tofsuppres's, andthe width of the channel being materially less than one saidwavele'ngth, said resonators being mounted substantiallyin a"single plane perpendic'larto the ldirection of propagation'of the waves to'befltered" whereby' narrowness ofl'band is prornoted, and the resonators^ being arranged in parallel"r`e lation'toone another and spaced'apart by material open spacestherebetween for the 4passage of'waves not to 'be suppressed, but notfarther apart than one-lialf'ofsaid wavelength Vto promote suppressionof the 'said `principal frequencysubs'tantially continuously over the entirerray including said open spaces.

` References Cited in the file of this patent UNITED STATES' PATENTS 

